Use credentials object directly when creating SSHLauncher

We found a weird bug where the SSHLauncher(String, int, String, String,
String, String) constructor was setting the credentials to null instead
of a valid (already created) credentials.

The SSHLauncher constructor being used is:
https://github.com/jenkinsci/ssh-slaves-plugin/blob/ssh-slaves-1.6/src/main/java/hudson/plugins/sshslaves/SSHLauncher.java#L249

To handle these broken slaves, I had to add a null check for credentials
in the returned JSON.